added extra check for save internal link extensions. (for use with

added extra check for save internal link extensions. (for use with
magicHTML)
Maybe this is the wrong fix but in case of iframes tags with a cid link to an
internal entity it's possible that stupid users download attached virus
entities because the iframe src pointed to a filename with .scr, .exe, .bat or
other extension. This patch only allow the following hardcoded save extensions:
gif, jpeg, jpg, png, bmp.

To do:
notify user in case of invalid extensions.
totally remove iframes because I think in most cases (virus) they are unsave.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@3468 7612ce4b-ef26-0410-bec9-ea0150e637f0